1. Field of the Invention
The present invention relates generally to an optical fiber amplifier, and in particular, to a gain flattening device for an optical fiber amplifier.
2. Description of the Related Art
Wavelength Division Multiplexing (WDM) optical transmission requires an optical transmitter stabilized in wavelength and output power, a multiplexer (MUX) for multiplexing a plurality of wavelengths, and an optical fiber amplifier having a uniform gain distribution across a multiplexed wavelength band. A requirement for an Erbium-Doped Fiber Amplifier (EFDA) designed for Dense Wavelength Division Multiplexing (DWDM) is to have the same gain and noise characteristic at any wavelength. If optical signals have different gain and noise characteristics, even though multiplexed with the same intensity at any wavelength, the differences in gain and noise are accumulated after multi-stage amplification. This causes a signal-to-noise ratio (SNR) at a particular wavelength to decrease. As a result, a receiver may detect optical signals with a larger error rate variation across the spectrum of wavelengths. In this context, flattening the gain of the EDFA is very desirable in WDM optical transmission. In order to flatten the gain of the EDFA, an interference filter, a chirped Bragg grating, or a long period fiber grating is usually adopted as an end filter for the EDFA.
FIG. 1 illustrates a conventional EDFA having a gain flattening filter. The EDFA amplifies an optical signal that is propagated along an erbium-doped fiber 150 by population inversion of erbium ions. The EDFA includes an optical fiber 110 as a transmission medium. First and second optical isolators 120,180 are included for isolating any feedback light. The erbium-doped fiber 150 is utilized for amplifying optical signals. First and second pumping light sources 140,170 are included for outputting pumping light that excites erbium ions in the erbium-doped fiber 150. Further included, first and second optical couplers 130,160 for coupling the pumping light to the optical fiber 110, and a gain flattening filter 190.
In FIG. 1, a 9-channel optical signal is amplified in the EDFA and it has a non uniform gain distribution across the channels. This implies that the EDFA has a different gain at varying wavelengths. Today's optical communication systems transmit/receive a plurality of channels via one optical fiber in WDM and thus the wavelength spacing between channels is reduced due to limited wavelength bands. Accordingly, if the light intensities of channels are not uniform, the channels are highly susceptible to data loss due to noise or inter-channel interference.
The gain flattening filter 190 at the output of the EDFA flattens the wavelength-gain curve of the EDFA. Long period fiber gratings 195 in the gain flattening filter 190 have a Gaussian-like function gain curve in which a peak value is observed at the central frequency. If the transmitted optical signal uses a narrow wavelength band, a single long period fiber grating 195 is enough to flatten the gain. In the case of a wide wavelength band, a plurality of long period fiber gratings 195 are used in combination.
The conventional EDFA, however, has a shortcoming in that the use of the gain flattening filter 190 as an end filter reduces the gain of the EDFA rather than when it is used as a midway filter. Moreover, since the long period fiber gratings 195 are sensitive to temperature changes, the gain characteristics of the EDFA may deteriorate depending on changes in the ambient environment. Further, if the EDFA uses a reflection filter such as an interference filter and Bragg gratings as a midway filter, it is very difficult to block optical signals reflected from the reflection filter. While an optical isolator can block the reflected light, it is very expensive and thus the cost of the EDFA is increased.